Exhaust gas discharged from automobile internal combustion engines such as gasoline engines or diesel engines contains components such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx).
Consequently, internal combustion engines are typically provided with an exhaust gas purification device for decomposing and removing these components, and these components are substantially decomposed by an exhaust gas purification catalyst installed in this exhaust gas purification device. An example of such an exhaust gas purification catalyst is a NOx storage-reduction catalyst.
In a lean atmosphere, this NOx storage-reduction catalyst converts nitrogen oxides present in exhaust gas to NO2 and stores NO2, and reduces stored NO2 to nitrogen (N2) in stoichiometric and rich atmospheres, thereby altering the atmosphere of exhaust gas between lean, stoichiometric, and rich and making it possible to purify exhaust gas, for example, exhaust gas containing NOx, with high efficiency.
In general, it is preferable that NOx storage-reduction catalysts have high NOx storage performance. This is because, the amount of stored NO2 and the storage rate thereof in a lean atmosphere increase as NOx storage capacity becomes higher, and as a result thereof, both the amount of purified exhaust gas containing NOx in particular and the purification rate thereof increase. Thus, studies are being conducted on NOx storage-reduction catalysts demonstrating improved NO2 storage performance in particular.
The exhaust gas purification catalyst of Patent Document 1 has a substrate, a first catalyst layer containing rhodium and formed on the substrate, and a second catalyst layer containing platinum and formed on the first catalyst layer, and the second catalyst layer contains a support on which is loaded a first metal oxide containing a first metal element selected from the group consisting of alkaline metals and alkaline earth metals, and a second metal oxide containing a second metal element selected from the group consisting of rare earth metals. In this exhaust gas purification catalyst of Patent Document 1, it is described that the first and second metal oxides contained in the second catalyst layer have the ability to release sulfur, and that decreases in catalytic activity of precious metals are inhibited as a result thereof.
The exhaust gas purification catalyst of Patent Document 2 is provided with a catalyst layer, a plurality of voids having an elongated shape are present in this catalyst layer, and the mode of the frequency distribution relating to the aspect ratio of a cross-section of these voids is 2 or more. In this exhaust gas purification catalyst of Patent Document 2, it is described to the effect that the plurality of voids having an elongated shape present in the catalyst layer are interconnected, and that this results in an increase in the number of gas diffusion paths.